Active Steady State Research Articles

Experimental bifurcation analysis of the cobalt/phosphoric acid electrochemical oscillator

The dynamical behaviour of the I-E polarization curves of the Co 1 M H 3PO 4 electrochemical oscillator is studied as the potential varies across the active/passive transition of the Co electrode. An oscillatory potential region exists between the two stable steady-states where a hysteresis loop is formed by the sudden decrease (Co passivation) and increase (Co activation) of the current as the potential is scanned forward and backward. In the present study an attempt is made towards an experimental determination of the bifurcation structure of the Co 1 M H 3PO 4 system. Transition from the active steadystate (SS1) to the oscillatory state (OSC), and vice versa, is accompanied by a hysteresis and occurs at critical potential values via a generalised Hopf bifurcation. The identification of the generalised Hopf bifurcation is based on the presence of hysteresis, the sudden appearance of full amplitude oscillations during the transition SS1 → OSC and on the fact that the transition OSC → SS1 occurs with oscillations of a finite amplitude. On the other side of the loop, transition from the oscillatory region to the passive steady-state (SS2) goes via a saddle-type bifurcation. The period lengthening during the transition OSC → SS2, the existence of small hysteresis and the fact that oscillations appear and disappear with almost their full amplitude ensure a saddle-type bifurcation. Some of the bifurcation features are discussed on the basis of the surface processes taking place at the Co electrode and compared with the Fe 2 M H 2SO 4 electrochemical oscillator that was previously investigated. Potential and current perturbation experiments applied close to bifurcation points indicate that the Co 1 M H 3PO 4 system does not exhibit excitability in the corresponding regions where Fe 2 M H 2SO 4 does. Micrographs taken by the scanning electron microscope and examination of the chloride effect on the dynamical behaviour of the Co 1 M H 3PO 4 system support further the relation between the experimental findings and the mechanism leading to the non-linear behaviour across the active-passive transition.

Adsorption calorimetry : An overview of its practice and applications

Chemisorptions and catalyzed reactions, like any chemical reaction, are associated with changes of enthalpy and can therefore be studied by means of calorimeters. Many calorimeters, operating on different principles, have indeed been used for this purpose, as it will be attempted to show in a brief review (ref. 1). In our opinion, however, heat-flow calorimeters are particularly convenient for these studies (ref. 2). They offer a number of advantages which will be illustrated by means of selected examples. Heat-flow calorimetry, preferably in association with other physico-chemical or physical techniques, may be used to describe the surface properties of a solid. It will be shown that, for instance, differential heats of adsorption of ammonia may serve to quantitatively describe surface acidity in small-pore zeolite, e.g. H-ZSM5, and thus to complement the information provided by i.r. spectroscopy (ref. 3). Information on the bonds energy, deduced from calorimetric data, is needed to achieve a theoretical description of the adsorbate-adsorbent bond. It will be shown, for instance, that, in the case of the adsorption of hydrogen on nickel-copper alloys, a correlation between heats of adsorption and surface magnetic properties can be demonstrated. The correlation indicates that the energy of the bond between adsorbed hydrogen and nickel atoms is regulated by the electron density of states, near the Fermi level, for the metal surface (ref. 4). Adsorption of one, at least, of the reagents is a necessary step in all heterogeneously catalyzed reactions. Calorimetric investigations of the adsorptions of the pure reagents on the catalysts may therefore provide some information on the catalytic reaction itself. It appears, for instance, that there exists an inverse linear correlation between the activity of silver catalysts for the epoxidation of ethylene and the bonding energy of oxygen at their surface (ref. 5). The change of enthalpy associated with the reaction is, of course, not modified by the presence of the catalyst. Therefore, thermochemical cycles, based on experimentally determined heats of adsorption of interaction of reagents, introduced successively at the catalyst surface, may give indication on the most probable reaction mechanism, in the case of “simple” catalytic reactions. The recorded heat of reaction, however, may differ from the expected one when secondary reactions, eventually leading to the catalyst activation or deactivation, take place. The calorimetric study of the catalytic reaction, when a steady state of activity is not yet established, may therefore give information on the catalysts evolution or even on the reaction mechanism. These applications of calorimetry will be illustrated by various studies of the combustion of carbon monoxide on divided nickel oxide catalysts at 300 and 473 K (ref. 6). Heat-flow calorimeters are also used for kinetic studies. Several examples will be presented to show how reaction orders, kinetic laws and, in favourable cases, activity spectra of the catalysts surface can be determined (ref. 1). In the case of fast processes, deconvolution of the calorimetric data must be achieved (ref. 7).

Distribution and fate of 54Mn in the monkey: studies of differnnt parts of the central nervous system and other organs.

The fate and distribution of isotopic manganese administered as a single carrier-free dose of 200 muCi of maleate-(54)Mn to 12 rhesus monkeys was studied at different time periods from the 6th hr to the 278th day. Whole-body activity was measured, and all body organs and tissues and different parts of the central nervous system (CNS) were evaluated for specific activity, exponential analysis, and relative retention. Exponential analysis revealed a pattern of discharge with a fast and a slow component for the whole body and for many of the viscera. All parts of the CNS and, to a lesser degree, the thyroid and muscle showed an almost steady state of activity after the initial uptake. While the whole body and most organs and tissues appeared to discharge their radioactivity with the passage of time, first rapidly and then gradually, the CNS, endocrine glands, and muscle tissues showed persistent levels of specific activity. All components of the brain exhibited increasing relative retention, the lentiform nucleus and the cerebellum showing this more. It is suggested that the selective vulnerability of the brain in manganese miners might result from this inability on the part of the CNS to discharge the (54)Mn with time. This investigation confirms and amplifies our earlier similar study on the rat.

Cortical D.C. changes incident to midline thalamic stimulation

1. 1. Cortical D.C. changes occur incident to stimulation of the mid-line thalamus of rabbit and cat. 2. 2. At stimulating frequencies which elicit recruiting a negative D.C. shift ordinarily occurs in the rabbit. In the cat there is no shift or a minor positive one may occur. 3. 3. In the rabbit with high frequency stimulation a negative D.C. shift results which may obtain an amplitude of 1.5 mV. and persist for several seconds into the post-stimulatory period. In the cat there is also a negative shift, but it is usually less manifest than the rabbit. 4. 4. The D.C. shift with low frequency stimulation results from summation of after-effect. At high frequency stimulation the D.C. shift is accounted for by summation of transients as well as summation of after-effects and is regarded as a steady state of activity in dendrites. 5. 5. Prolonged stimulation (10–15/sec) may introduce other effects which occasion the return of the D.C. shift to the pre-stimulatory base line and post-stimulatory positivity. 6. 6. The after-effect following the initial response of a recruiting series is associated with increased excitability. The negative D.C. shift following brief repetitive stimulation and the positive one following prolonged (10–15 sec.) stimulation are associated with decreased excitability for a test recruiting series. 7. 7. Intense repetitive stimulation in the relay nuclei of the thalamus may occasion cortical paroxysm on a base line indicative of positive D.C. shift, whereas corresponding stimulation in the mid-line thalamus has been shown to relate to negative D.C. shift. At present no explanation can be offered for this difference.

Lactic and pyruvic acid relations in contracting mammalian muscle.

Tetanic contraction of mammalian muscle under essentially anaerobic conditions was found to result in marked increase in the pyruvic acid content as well as in the lactic acid content. The increase in lactic acid content was proportionately greater than in the pyruvic acid content. Repeated single twitches at a rate of 1/sec., continued long enough to produce a steady state, resulted in only a slight increase in pyruvic acid content, with a marked decrease in the ratio of pyruvic to lactic acid. The pyruvic acid formed during the steady state of activity appears not to pass into the blood stream.